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Isolated tubule perfusion is an experimental technique in which a dissected renal tubule segment is supplied internally with a controlled solution while a separate bath contacts its outside. Fine micropipettes deliver and collect tubular fluid. Keeping luminal and external conditions separate lets investigators test transport across the epithelium of one identified segment.

For example, isolated rabbit thin descending limb segments were perfused in a 1970 study while bath osmolality was changed. Concentration of the collected fluid increased mainly through water leaving, rather than substantial salt entering. The apparatus converted a question about whole-kidney concentration into a direct test of one segment's membrane permeability.^[ref]

A simple mass balance estimates net water transport. If inflow and collected outflow are $Q_{\mathrm{in}}$ and $Q_{\mathrm{out}}$ for a segment of length $L$, then outward volume transport per unit length isassuming no leak or accumulation. Hypothetical flows of $20$ and $18nL/min$ through a $2\mathrm{mm}$ segment give $J_v=1\mathrm{nL}\mathrm{mi}{\mathrm{n}}^{-1}\mathrm{m}{\mathrm{m}}^{-1}$. A solute requires a separate balance involving concentration as well as flow.

Isolation provides experimental control while removing the segment from its normal blood supply, neighboring cells and systemic signals. It therefore complements renal micropuncture in an intact kidney: neither preparation alone supplies every part of the physiological explanation.

Maurice B. Burg was a researcher in renal physiology who developed experimental methods for studying individual nephron segments outside the intact kidney. Isolated tubule perfusion made it possible to relate transport properties to a particular anatomical segment, rather than observing only the final composition of urine.^[ref]

The 1966 method paper by Maurice B. Burg and collaborators described preparations from single rabbit nephrons. Supplying fluid through an isolated renal tubule while controlling its surrounding solution allows the luminal and external conditions to be varied separately.^[ref]

A useful experiment is to change the surrounding osmolality and compare entering and collected tubular fluid. A change in collected volume can reveal net water movement; changes in solute content address transport of dissolved substances. Isolating the segment constrains where the measured exchange occurred. The preparation simplifies the system while retaining an organized living epithelium, an important middle scale between a whole organ and an isolated protein.